This invention relates generally to the field of generating activated gas containing ions, free radicals, atoms and molecules and to apparatus for and methods of processing materials with activated gas.
Plasma discharges can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing solid materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
For example, some applications require the use of ions with low kinetic energy (i.e. a few electron volts) because the material being processed is sensitive to damage. Other applications, such as anisotropic etching or planarized dielectric deposition, require the use of ions with high kinetic energy. Still other applications, such as reactive ion beam etching, require precise control of the ion energy.
Some applications require direct exposure of the material being processed to a high density plasma. One such application is generating ion-activated chemical reactions. Other such applications include etching of and depositing material into high aspect ratio structures. Other applications require shielding the material being processed from the plasma because the material is sensitive to damage caused by ions or because the process has high selectivity requirements.
Plasmas can be generated in various ways including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Parallel plates are typically used for electrostatically coupling energy into a plasma. Induction coils are typically used for inducing current into the plasma. Microwave discharges are achieved by directly coupling microwave energy through a microwave-processing window into a discharge chamber containing a gas. Microwave discharges are advantageous because they can be used to support a wide range of discharge conditions, including highly ionized electron cyclotron resonant (ECR) plasmas.
RF discharges and DC discharges inherently produce high energy ions and, therefore, are often used to generate plasmas for applications where the material being processed is in direct contact with the plasma. Microwave discharges produce dense, low ion energy plasmas and, therefore, are often used to produce streams of activated gas for xe2x80x9cdownstreamxe2x80x9d processing. Microwave discharges are also useful for applications where it is desirable to generate ions at low energy and then accelerate the ions to the process surface with an applied potential.
However, microwave and inductively coupled plasma sources require expensive and complex power delivery systems. These plasma sources require precision RF or microwave power generators and complex matching networks to match the impedance of the generator to the plasma source. In addition, precision instrumentation is usually required to ascertain and control the actual power reaching the plasma.
RF inductively coupled plasmas are particularly useful for generating large area plasmas for such applications as semiconductor wafer processing. However, prior art RF inductively coupled plasmas are not purely inductive because the drive currents are only weakly coupled to the plasma. Consequently, RF inductively coupled plasmas are inefficient and require the use of high voltages on the drive coils. The high voltages produce high electrostatic fields that cause high energy ion bombardment of reactor surfaces. The ion bombardment deteriorates the reactor and can contaminate the process chamber and the material being processed The ion bombardment can also cause damage to the material being processed.
Faraday shields have been used in inductively coupled plasma sources to contain the high electrostatic fields. However, because of the relatively weak coupling of the drive coil currents to the plasma, large eddy currents form in the shields resulting in substantial power dissipation. The cost, complexity, and reduced power efficiency make the use of Faraday shields unattractive.
It is therefore a principle object of this invention to provide a source of activated gas that uses a high efficiency RF power coupling device which couples power into a plasma without the use of conventional RF or microwave generators and impedance matching systems.
It is another principle object of this invention to provide a source of activated gas for materials processing where there is no significant energetic ion bombardment within the process reactor and where long-term operation can be sustained using chemically reactive gases without damage to the source and without production of contaminant materials.
It is another principle object of this invention to provide a source of activated gas in which either a metal, a dielectric, or a coated metal (e.g. anodized) can be used to form the source chamber.
A principal discovery of the present invention is that switching semiconductor devices can be used to efficiently drive the primary winding of a power transformer that couples electromagnetic energy to a plasma so as to form a secondary circuit of the transformer. It is another principal discovery of this invention that an inductively-driven toroidal plasma source can be constructed with a metallic plasma chamber.
Accordingly, the present invention features an apparatus for dissociating gases that includes a plasma chamber. The plasma chamber may be formed from a metallic material such as aluminum or may be formed from a dielectric material such as quartz. The metallic material may be a refractory metal. The apparatus may include a process chamber that is coupled to the plasma chamber and positioned to receive reactive gas generated by a plasma in the plasma chamber.
The apparatus also includes a transformer having a magnetic core surrounding a portion of the plasma chamber and having a primary winding. One or more switching semiconductor devices are directly coupled to a voltage supply and have an output coupled to the primary winding of the transformer. The output of the one or more switching semiconductor devices may be directly coupled to the primary winding of the transformer. The one or more switching semiconductor devices may be switching transistors. The voltage supply may be a line voltage supply or a bus voltage supply.
The apparatus may include a free charge generator which assists the ignition of a plasma in the chamber. In a preferred embodiment, an electrode is positioned in the chamber to generate the free charges. In another preferred embodiment, an electrode is capacitively coupled to the chamber to generate the free charges. In another preferred embodiment, an ultraviolet light source is optically coupled to the chamber to generate the free charges.
The apparatus may include a circuit for measuring electrical parameters of the primary winding and of the plasma. The circuit measures parameters such as the current driving the primary winding, the voltage across the primary winding, the bus supply voltage, the average power in the primary winding, and the peak power in the primary winding. A power control circuit may be coupled to the circuit for measuring electrical parameters of the primary winding and the plasma. The power control circuit regulates the current flowing through the primary windings based upon a measurement of the electrical properties of the primary winding and of the plasma and from a predetermined set point representing a desired operating condition.
The present invention also features a method for dissociating gases. The method includes providing a chamber for containing a gas at a pressure. The pressure may be substantially between 1 mtorr and 100 torr. The gas may comprise a noble gas, a reactive gas or a mixture of at least one noble gas and at least one reactive gas. The method also includes providing a transformer having a magnetic core surrounding a portion of the chamber and having a primary winding.
In addition, the method includes directly coupling one or more switching semiconductor devices to a voltage supply, which may be a line voltage supply or a bus voltage supply. The one or more switching semiconductor devices are also coupled to the primary winding of the transformer so that they generate a current that drives the primary winding. The one or more switching semiconductor devices may be directly coupled to the primary winding of the transformer.
The method also includes inducing a potential inside the plasma chamber with the current in the primary winding of the transformer. The magnitude of the induced potential depends on the magnetic field produced by the core and the frequency at which the switching semiconductor devices operate according to Faraday""s law of induction. The potential forms a plasma which completes a secondary circuit of the transformer. The electric field of the plasma may be substantially between 1-100 V/cm. The method may include providing an initial ionization event in the chamber. The initial ionization event may be the application of a voltage pulse to the primary winding or to an electrode positioned in the plasma chamber. The initial ionization event may also be exposing the chamber to ultraviolet radiation.
The method may include the step of measuring electrical parameters of the primary winding and of the plasma including one or more of the current driving the primary winding, the voltage across the primary winding, the bus voltage, the average power in the primary winding, and the peak power in the primary winding. In addition, the method may include the step of determining an output of the one or more switching semiconductor devices from the measurement of the electrical parameters of the primary winding, the plasma, and from a predetermined set point representing a desired operating condition.
The present invention also includes a method for cleaning a process chamber. The method includes providing a plasma chamber that is coupled to the process chamber. The plasma chamber contains a reactive gas at a pressure. A transformer is provided having a magnetic core surrounding a portion of the plasma chamber and having a primary winding. One or more switching semiconductor devices is directly coupled to a voltage supply for generating a current that drives the primary winding of the transformer.
In addition, the method includes inducing a potential inside the plasma chamber with the current in the primary winding. The magnitude of the induced potential depends on the magnetic field produced by the core and the frequency at which the switching semiconductor devices operate according to Faraday""s law of induction. The potential forms a plasma which completes a secondary circuit of the transformer. The method also includes directing chemically active species such as atoms, molecules and radicals generated in the plasma from the plasma chamber into the process chamber thereby cleaning the process chamber.
The present invention also includes a method for generating reactive gases. The method includes providing a plasma chamber that is coupled to the process chamber. The plasma chamber contains a reactive gas at a pressure. A transformer is provided having a magnetic core surrounding a portion of the plasma chamber and having a primary winding. One or more switching semiconductor devices is directly coupled to a voltage supply for generating a current that drives the primary winding of the transformer.
In addition, the method includes inducing a potential inside the plasma chamber with the current in the primary winding. The magnitude of the induced potential depends on the magnetic field produced by the core and the frequency at which the switching semiconductor devices operate according to Faraday""s law of induction. The potential forms a plasma which completes a secondary circuit of the transformer. The method also includes generating reactive gas in the plasma.
The present invention also features an apparatus for generating ions. The apparatus includes a plasma chamber that may be formed from a metallic material such as a refractory metal. An orifice is positioned in the chamber for directing ions generated by the plasma. A process chamber may be coupled to the orifice in the plasma chamber and adapted to receive ions generated by the plasma. Accelerating electrodes may be positioned in the process chamber for accelerating ions generated by the plasma.
The apparatus includes a transformer having a magnetic core surrounding a portion of the plasma chamber and having a primary winding. One or more switching semiconductor devices are directly coupled to a voltage supply, which may be a line voltage supply or a bus voltage supply, and have an output coupled to the primary winding of the transformer. In operation, the one or more switching semiconductor devices drives current in the primary winding of the transformer. The current induces a potential inside the chamber that forms a plasma which completes a secondary circuit of the transformer. Ions are extracted from the plasma through the orifice. The ions may be accelerated by the accelerating electrodes.
The present invention also features another apparatus for dissociating gases. The apparatus includes a plasma chamber that comprises an electrically conductive material such as aluminum and at least one dielectric region that prevents induced current flow in the chamber. The plasma chamber may include a plurality of dielectric regions separating at least two regions of the plasma chamber. The dielectric region may comprise a dielectric coating on at least one mating surface of the chamber. The plasma chamber may also include cooling channels for passing a fluid that controls the temperature of the chamber.
In addition, the apparatus includes a transformer having a magnetic core surrounding a portion of the plasma chamber and having a primary winding. The apparatus also includes a power supply that has an output electrically connected to the primary winding of the transformer. The power supply drives current in the primary winding that induces a potential inside the chamber that forms a plasma which completes a secondary circuit of the transformer. The power supply may comprise one or more switching semiconductor devices that are directly coupled to a voltage supply and that have an output coupled to the primary winding of the transformer. The voltage supply may comprise a line voltage supply or a bus voltage supply.
The apparatus may include a means for generating free charges that assists the ignition of a plasma in the chamber. In a preferred embodiment, an electrode is positioned in the chamber to generate the free charges. In another preferred embodiment, an electrode is capacitively coupled to the chamber to generate the free charges. In another preferred embodiment, an ultraviolet light source is optically coupled to the chamber to generate the free charges.